In the drilling of wells for oil and gas by the rotary method, it is common to use a circulating fluid which is pumped down to the bottom of the well through a drill pipe, where the fluid emerges through ports in the drilling bit. The fluid rises to the surface in the annular space between the drill pipe and the walls of the hole, and at the surface it is treated to remove cuttings and the like to prepare it for recirculation into the drill pipe. The circulation is substantially continuous while the drill pipe is rotated.
The present invention pertains to oil-base drilling fluids or oil-base muds which includes water-in-oil (invert) emulsions as well as oil-base fluids containing only small amounts or no emulsified water.
An important feature of well working fluids of the class described is their ability to resist filtration. In most instances, when they are in actual use, whether as drilling fluids, packer fluids, fracturing or completion fluids, the well working fluids is in contact with a more or less permeable formation, such as, for example, sandstone, sandy shale and the like, with an effctive balance of pressure such that the fuid tends to be forced into the permeable formation. When a well working fluid is deficient in its ability to resist filtration, then the solids in the fluid are held back by the permeable formation and build up as a filter cake or sludge on its surfaces, while the liquid per se of the well working fluid filters into the permeable formation. The filter cake or sludge thus formed is generally very undesirable. Moreover, the loss of oil to the formation is very expensive, not only because of the cost of the oil itself, but also due to the cost of maintaining the properties and composition of the fluid.
Various additives have been used or suggested for use as fluid loss additives to prevent or decrease this loss of fluid by filtration from oil-base muds. Some of the materials used or proposed for use for this purpose are asphalt and various modified alphaltic materials, various organic modified humic acid or lignite derivatives, and various polymers.